Endless power transmission belts each having a plurality of longitudinally extending ribs, and often referred to as ribbed belts, are in wide use throughout industry. One of the problems associated with each of such ribbed belts is that the belt wears unevenly and fails prematurely because of poor operative engagement of the belt with associated sheaves of a drive system. In general, this has been due to two factors one being that the sheaves used with each of such ribbed belts have projections in which the apexes thereof extend between the belt ribs and engage the main belt body, due to lack of relief space between belt ribs, providing high load concentrations at their points of engagement which result in excessive heat buildup, premature wear, and failure of the entire belt. The second factor has been that the ribs of such ribbed belts extend too far into the grooves of their associated sheaves often bottoming out, long before a satisfactory belt life has been achieved, and again causing excessive heat buildup, premature wear, and belt failure.